Enter the room that separates the security offices from Bravo spawn--this room is usually called the "broom closet". Find a bucket sitting in front of a cart just before you get to the security offices. If you try to take cover on the bucket, it should force you to take cover on the cart. When this happens, look up and over the cart, then release LT and walk forward. You should then be on top of the cart. If you stand up, you will be able to see upstairs through the ceiling. This is useful to watch for approaching enemies, you can toss grenades, and you cannot shoot through it. If an enemy sees you, crouch back down or he will shoot you in the head.